1. Field of the Invention
The invention relates to a process for producing polyurethane foams using the one-shot foaming process. The invention specifically relates to using a salt of a tertiary amine and a carboxylic acid with hydroxyl functionality as a catalyst for promoting reactions involved in the production of one-shot polyurethanes, particularly flexible polyurethane foams.
2. Background
Polyurethane foams are produced by reacting a polyisocyanate with compounds containing two or more active hydrogens. The active hydrogen-containing compounds are typically polyols, primary and secondary polyamines, and water. Two major reactions take place among these reactams during the preparation of a polyurethane foam. These reactions must proceed simultaneously and at a competitively balanced rate during the process in order to yield a polyurethane foam with desired physical characteristics.
Reaction between the isocyanate and the polyol or polyamine, usually referred to as the gel reaction, leads to the formation of a polymer of high molecular weight. This reaction is predominant in foams blown exclusively with low boiling point organic compounds. The progress of this reaction increases the viscosity of the mixture and generally contributes to crosslink formation with polyfunctional polyols. The second major reaction occurs between the isocyanate and water. This reaction adds to urethane polymer growth, and is important for producing carbon dioxide gas which promotes foaming. As a result, this reaction often is referred to as the blow reaction.
Both the gel and blow reactions occur in foams blown partially or totally with carbon dioxide gas. In fact, the in-situ generation of carbon dioxide by the blow reaction plays an essential part in the preparation of "one-shot", water blown polyurethane foams. Water-blown polyurethane foams, particularly flexible foams, are produced by both molded and slab foam processes.
In order to obtain a good urethane foam structure, the gel and blow reactions must proceed simultaneously and at optimum balanced rates. For example, if the carbon dioxide evolution is too rapid in comparison with the gel reaction, the foam tends to collapse. Alternatively, if the gel extension reaction is too rapid in comparison with the blow reaction generating carbon dioxide, foam rise will be restricted, thus resulting in a high-density foam. Also, poorly balanced crosslinking reactions will adversely impact foam stability. In practice, the balancing of these two reactions is controlled by the nature of the promoters and catalysts, generally amine and/or organometallic compounds, used in the process.
Flexible and rigid foam formulations usually include a polyol, a polyisocyanate, water, optionally a low boiling (highly volatile) organic blowing agent, a silicone type surfactant, and catalysts. Flexible foams are generally open-celled materials, while rigid foams usually have a high proportion of closed cells.
Generally, catalysts for producing polyurethanes are of two general types: tertiary amines (mono and poly) and organo-tin compounds. Organometallic tin catalysts predominantly favor the gelling reaction; while amine catalysts exhibit a more varied range of blow/gel balance. Using tin catalysts in flexible foam formulations also increases the quantity of closed cells contributing to foam tightness. Tertiary amines also are effective as catalysts for the chain extension reaction and can be used in combination with the organic tin catalysts. For example, in the preparation of flexible slabstock foams, the "one-shot" process has been used wherein triethylenediamine is employed for promoting the water-isocyanate reaction and the cross-linking reaction; while an organic tin compound is used in synergistic combination to promote the chain extension reaction.
Most tertiary amines used for the catalysis of polyurethane foam forming reactions are of the fugitive type. Fugitive amines are designated as such because they do not react into the urethane polymer matrix and remain as low molecular weight compounds in the polymer. Many prior art fugitive amines impart a strong amine odor to the polyurethane foam and may present significant safety problems. The fugitivity of amines results in the emission of fumes from hot foam in both molded foam and slabstock foam processes. Airborne amine vapors can be an industrial hygiene problem in foam production plants. A particular effect of the amine vapor is glaucopsia also known as blue-haze or halovision. It is a temporary disturbance of the clarity of vision. There is increasing demand in the foam production industry for low fugitivity catalysts.
Amines which have a functional group capable of reacting with the isocyanate are available. These amines are bound to the polymer matrix during the reaction. Unfortunately, their catalytic activity normally is limited as compared to the fugitive amines.
Flexible polyurethane foams are commercially prepared as slabstock foam or in molds. Although some slabstock foam is produced by pouring the mixed reactants in large boxes, the predominant industrial process is the continuous production by deposition of the reacting mixture on a paper lined conveyor. The foam rises and cures as the conveyor advances and the foam is cut into large blocks as it exits the foam machine. Some of the uses of flexible slabstock polyurethane foams include: furniture cushions, bedding, and carpet underlay. A particular problem occurs when slabstock foam is produced by the continuous process on a machine with a short conveyor. The formulation has to be highly catalyzed in order to be sufficiently cured when the foam reaches the cutting saw. However, the initiation of the reaction must be delayed to allow uniform laydown of the reacting mixture. In such situations, delayed action catalysts potentially can be used to achieve the required reactivity profile.
The process for making molded foams typically involves the mixing of the starting materials with polyurethane foam production machinery and pouring the reacting mixture, as it exits the mix-head, into a mold. The principal uses of flexible molded polyurethane foams are: automotive seats; automotive headrests and armrests; and also in furniture cushions. Some of the uses of semi-flexible foams include automotive instrument panels, energy managing foam, and sound absorbing foam.
Modern molded flexible polyurethane foam production processes such as those used in Just-in-Time (JIT) supply plants have increased the demand for rapid demold systems. Gains in productivity and/or reduced part cost result from reduced cycle times. Rapid cure High Resilience (HR) molded flexible foam formulations typically achieve demold times of three minutes. This is accomplished by using one or a combination of the following: a higher mold temperature, more reactive intermediates (polyols and/or isocyanate), or increasing the quantity and/or the activity of the catalysts.
High reactivity molded polyurethane systems give rise to a number of problems, however. The fast initiation times require that the reacting chemicals be poured into a mold quickly. In some circumstances a rapid build-up of the viscosity of the rising foam causes a deterioration of its flow properties and can result in defects in the molded parts. Additionally, rapidly rising foam can reach the parting line of the mold cavity before the cover has had the time to close resulting in collapsed areas in the foam. In such situations, delayed action catalysts potentially can be used to improve the initial system flow and allow sufficient time to close the mold.
Another difficulty experienced in the production of molded foams, which is usually worse in the case of rapid cure foam formulations, is foam tightness. Foam tightness is caused by a high proportion of closed cells at the time the molded foam part is removed from the mold. If left to cool in that state, the foam part will generally shrink irreversibly. A high proportion of open cells also are required if the foam is to have the desired high resiliency. Consequently, foam cells have to be opened either by physically crushing the molded part or inserting it in a vacuum chamber. Many strategies have been proposed, both chemical and mechanical, to minimize the quantity of closed cells at demold.
The principal uses of rigid polyurethane foam are: pour-in-place insulation foams for refrigeration applications, transportation applications, and metal doors, boardstock insulation, and sprayed insulation. In rigid foam applications, delayed action catalysts are used for the same reasons needed in flexible foam molding, to delay the initial system reactivity while offering the short cure times required for fast productions cycles.
Delayed action catalysts used in the above-described processes are usually simple amine salts of a tertiary amine and a carboxylic acid such as formic acid, acetic acid, or 2-ethylhexanoic acid (J. Cellular Plastics, p. 250-255, September/October, 1975). The salts are not catalytically active and, as a consequence, the amines do not activate the reaction until the salt is dissociated by the increasing temperature of the reacting mixture. Unfortunately, using carboxylic acid blocked amine catalysts generally has a tightening effect on the foam (see U.S. Pat. Nos. 3,385,806, 4,701,474, and 4,785,027).
Delayed action catalysts find their main application in the manufacture of molded flexible polyurethane foam pans. In such applications, it is desirable to make the molding time as short as possible ("rapid demold"), but the onset of the reaction must be delayed so that the viscosity increase accompanying the reaction does not jeopardize proper mold filling.
One problem specific to the use of delayed action, acid-blocked catalysts, i.e., acid-amine salts, is the corrosion caused to the production equipment of the system by such materials. Foam machines usually produce foam by mixing the isocyanate with a mixture of the other components of the formulation either through high pressure impingement or by high speed stirring. The mixture of the ingredients, save the isocyanate, is collectively called the resin. The resin usually includes the polyol, water, silicone surfactant, and the catalysts. Delayed action catalysts are most conveniently incorporated into the resin directly or as a water/amine salt premix. The acid-blocked, amine salt catalysts often cause significant corrosion damage to the mixing and dispensing equipment used in urethane foam manufacture, particularly the pumps and mix-head.
There remains a need in the polyurethane industry for catalysts that have a delayed action; so as to delay the onset of the isocyanate-polyol reaction, referred to as the "initiation time", without adversely impacting the time to complete the reaction or cure, while avoiding some of the other problems common to known delayed action catalysts.
3. Description of Related Art
The use of acid-grafted polyether polyols as reactivity controllers for the production of polyurethane foams is disclosed in U.S. Pat. No. 4,701,474. Such acid-grafted polyether polyols purportedly reduce the reactivity of polyurethane foam formulations without the tightening effect which usually results from using carboxylic acid-amine salts. The number average molecular weight range claimed for the disclosed acid-grafted polyether polyols is 1,000 to 10,000.
Preparing polyurethane foams in the presence of polyether acids is disclosed in U.S. Pat. No. 4,785,027. The polyether acids are mono- or di-acids with the acid functional groups located at the ends of the polymer chains. The polyether chain is built from ethylene and/or propylene oxide to have repeating alkoxy groups. In the case of mono acids, the other terminal group can be an alkyl or hydroxyl function. The presence of the hydroxyl functional group is optional. Such polyether acids purportedly delay the initial reaction rate without increasing foam tightness observed with formic acid-amine salts.
In U.S. Pat. No. 4,366,084 the fuming of dimethylaminopropylamine (DMAPA) is reduced by blocking the amine with phenol. The reduction in fuming increases directly with the percent blocking. According to the patent, using the DMAPA-phenol salts at varied blocking ratios does not cause any deterioration in the air flow and compression set properties of the foam.
U.S. Pat. No. 5,179,131 discloses that the addition of mono- or dicarboxylic acids to polyurethane foam formulations made using polyisocyanate polyaddition polymer poly-dispersions results in a reduction in foam shrinkage. The functional groups attached to the acid are either alkyl or alkylene.
A process for making open-celled crosslinked foams is disclosed in U.S. Pat. No. 4,211,849. The crosslinker is a crystalline polyhydroxy compound having at least 3 hydroxy groups.
The use of the amine salts of tertiary amino-acids as delayed action catalysts in the production of polyurethanes is disclosed in U.S. Pat. No. 4,232,152.
The use of particular N-hydroxyalkyl quaternary ammonium carbonylate salts as delayed action catalysts for the production of polyurethane is disclosed in U.S. Pat. Nos. 4,040,992 and 4,582,861.
The use of particular aliphatic tertiary monoamines, and the carboxylic acid salts thereof as catalysts, in the production of polyurethane foam is disclosed in U.S. Pat. Nos. 4,450,246 and 4,617,286 and in Canadian Pat. 651,638. A variety of organic mono or dicarboxylic acids are disclosed. Canadian Pat. 651,638, in particular, describes preparing polyurethane foams from a isocyanate-terminated polytetramethyleneether or polypropyleneether polyurethane prepolymer and water, in the presence of an acid-amine salt. In certain examples, salts of the hydroxy-acid, citric acid and either N-methyl morpholine and triethylamine are specifically exemplified.